438 research outputs found
Superconductivity mediated by a soft phonon mode: specific heat, resistivity, thermal expansion and magnetization of YB6
The superconductor YB6 has the second highest critical temperature Tc among
the boride family MBn. We report measurements of the specific heat,
resistivity, magnetic susceptibility and thermal expansion from 2 to 300 K,
using a single crystal with Tc = 7.2 K. The superconducting gap is
characteristic of medium-strong coupling. The specific heat, resistivity and
expansivity curves are deconvolved to yield approximations of the phonon
density of states, the spectral electron-phonon scattering function and the
phonon density of states weighted by the frequency-dependent Grueneisen
parameter respectively. Lattice vibrations extend to high frequencies >100 meV,
but a dominant Einstein-like mode at ~8 meV, associated with the vibrations of
yttrium ions in oversized boron cages, appears to provide most of the
superconducting coupling and gives rise to an unusual temperature behavior of
several observable quantities. A surface critical field Hc3 is also observed.Comment: 29 pages, 5 tables, 17 figures. Accepted for publication in Phys.
Rev.
Abrikosov flux-lines in two-band superconductors with mixed dimensionality
We study vortex structure in a two-band superconductor, in which one band is
ballistic and quasi-two-dimensional (2D), and the other is diffusive and
three-dimensional (3D). A circular cell approximation of the vortex lattice
within the quasiclassical theory of superconductivity is applied to a recently
developed model appropriate for such a two-band system [Tanaka et al 2006 Phys.
Rev. B 73, 220501(R); Tanaka et al 2007 Phys. Rev. B 75, 214512]. We assume
that superconductivity in the 3D diffusive band is "weak", i.e., mostly
induced, as is the case in MgB. Hybridization with the "weak" 3D diffusive
band has significant and intriguing influence on the electronic structure of
the "strong" 2D ballistic band. In particular, the Coulomb repulsion and the
diffusivity in the "weak" band enhance suppression of the order parameter and
enlargement of the vortex core by magnetic field in the "strong" band,
resulting in reduced critical temperature and field. Moreover, increased
diffusivity in the "weak" band can result in an upward curvature of the upper
critical field near the transition temperature. A particularly interesting
feature found in our model is the appearance of additional bound states at the
gap edge in the "strong" ballistic band, which are absent in the single-band
case. Furthermore, coupling with the "weak" diffusive band leads to reduced
band gaps and van Hove singularities of energy bands of the vortex lattice in
the "strong" ballistic band. We find these intriguing features for parameter
values appropriate for MgB.Comment: 11 pages, 14 figure
Observation of Magnetic Flux Generated Spontaneously During a Rapid Quench of Superconducting Films
We report observations of spontaneous formation of magnetic flux lines during
a rapid quench of YBaCuO films through T. This
effect is predicted according to the Kibble-Zurek mechanism of creation of
topological defects of the order parameter during a symmetry-breaking phase
transition. Our previous experiment, at a quench rate of 20K/sec, gave null
results. In the present experiment, the quench rate was increased to
\TEXTsymbol{>} 10 K/sec. Within experimental resolution, the dependence
of the measured flux on the cooling rate is consistent with the prediction
Specific heat of heavy fermion CePd2Si2 in high magnetic fields
We report specific heat measurements on the heavy fermion compound CePd2Si2
in magnetic fields up to 16 T and in the temperature range 1.4-16 K. A sharp
peak in the specific heat signals the antiferromagnetic transition at T_N ~ 9.3
K in zero field. The transition is found to shift to lower temperatures when a
magnetic field is applied along the crystallographic a-axis, while a field
applied parallel to the tetragonal c-axis does not affect the transition. The
magnetic contribution to the specific heat below T_N is well described by a sum
of a linear electronic term and an antiferromagnetic spin wave contribution.
Just below T_N, an additional positive curvature, especially at high fields,
arises most probably due to thermal fluctuations. The field dependence of the
coefficient of the low temperature linear term, gamma_0, extracted from the
fits shows a maximum at about 6 T, at the point where an anomaly was detected
in susceptibility measurements. The relative field dependence of both T_N and
the magnetic entropy at T_N scales as [1-(B/B_0)^2] for B // a, suggesting the
disappearance of antiferromagnetism at B_0 ~ 42 T. The expected suppression of
the antiferromagnetic transition temperature to zero makes the existence of a
magnetic quantum critical point possible.Comment: to be published in Journal of Physics: Condensed Matte
SO(5) superconductor in a Zeeman magnetic field: Phase diagram and thermodynamic properties
In this paper we present calculations of the SO(5) quantum rotor theory of
high-T superconductivity in Zeeman magnetic field. We use the spherical
approach for five-component quantum rotors in three-dimensional lattice to
obtain formulas for critical lines, free energy, entropy and specific heat and
present temperature dependences of these quantities for different values of
magnetic field. Our results are in qualitative agreement with relevant
experiments on high-T cuprates.Comment: 4 pages, 2 figures, to appear in Phys. Rev. B, see http://prb.aps.or
In-plane optical spectral weight transfer in optimally doped BiSrCaCuO
We examine the redistribution of the in-plane optical spectral weight in the
normal and superconducting state in tri-layer \bbb (Bi2223) near optimal doping
( = 110 K) on a single crystal via infrared reflectivity and spectroscopic
ellipsometry. We report the temperature dependence of the low-frequency
integrated spectral weight for different values of the cutoff
energy . Two different model-independent analyses consistently show
that for = 1 eV, which is below the charge transfer gap,
increases below , implying the lowering of the kinetic
energy of the holes. This is opposite to the BCS scenario, but it follows the
same trend observed in the bi-layer compound \bb (Bi2212). The size of this
effect is larger in Bi2223 than in Bi2212, approximately scaling with the
critical temperature. In the normal state, the temperature dependence of
is close to up to 300 K
Mixed-State Specific Heat of the Type-II Superconductor Nb0.77Zr0.23 in Magnetic Fields up to B c2
In order to document the behavior of the mean-field mixed-state specific heat of an isotropic. strongly type-II superconductor (i.e., with a large value of the Ginzburg parameterk), and to provide a basis for comparison with high-temperature superconductors, we measured the specific heatC of the alloy Nb0.77Zr0.23 withT c = 10.8K, B c2 (0) = 7.9T, in magnetic fieldsB = 0, 0.2, 1.0, 12, 2.0, 2.4. 3.0, 3.3. 4.0. 4.4, 4.8, 5.2, 6.0. 6.6, 7.2 and 10 T. The values of the upper critical fieldB c2 ( T), thermodynamic critical fieldB c (T), Ginzburg parameterk(T), and coefficient γ(B) = limT0(C(T. B)/T) are derived from the specific heat data and found to be in agreement with the GLAG theory in the dirty limit. The behavior of the mixed-state specific heat is analyzed in terms ofC el /T,∂(C el /T)/∂B, and∂(C el /T)/∂T vs. Tcurves, whereC el is the electronic contribution to the specific hea
Magnetic resonance at 41 meV and charge dynamics in YBa_2Cu_3O_6.95
We report an Eliashberg analysis of the electron dynamics in YBa_2Cu_3O_6.95.
The magnetic resonance at 41 meV couples to charge carriers and defines the
characteristic shape in energy of the scattering rate \tau^{-1}(T,\omega) which
allows us to construct the charge-spin spectral density I^2\chi(\omega,T) at
temperature T. The T dependence of the weight under the resonance peak in
I^2\chi(\omega,T) agrees with experiment as does that of the London penetration
depth and of the microwave conductivity. Als, at T=0 condensation energy, the
fractional oscillator strength in the condensate, and the ratio of gap to
critical temperature agree well with the data.Comment: 7 Pages, 3 Figures, accepted for publication in Europhysics Letter
Peculiarities of electronic heat capacity of thulium cuprates in pseudogap state
Precise calorimetric measurements have been carried out in the 7 - 300 K
temperature range on two ceramic samples of thulium 123 cuprates TmBa2Cu3O6.92
and TmBa2Cu3O6.70. The temperature dependence of the heat capacity was analyzed
in the region where the pseudogap state (PGS) takes place. The lattice
contribution was subtracted from the experimental data. The PGS component has
been obtained by comparing electronic heat capacities of two investigated
samples because the PGS contribution for the 6.92 sample is negligible. The
anomalous behavior of the electronic heat capacity near the temperature
boundary of PGS was found. It is supposed that this anomaly is due to
peculiarities in N(E) function where N is the density of electronic states and
E is the energy of carriers of charge.Comment: 12 pages, 3 Postscript figure
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